Letter and Symbol Series Letter and Symbol Series are a sequential order of letters, numbers or both arranged such a way that each term in the series is obtained according to some specific rules. These rules can be based on mathematical operations, place of letters in alphabetical order etc.Letter series is a logical arrangement of letters of English alphabet arranged in a specified pattern.In this section, a series of letters, groups of letters or combinations of letters and numbers is given. Each group or single element is called term. The terms of the series form a certain pattern. Candidates are required to identify this pattern and find the missing term(s) in the given series which will satisfy the pattern.